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Adventurer

Designer

Many thanks for your replies to my query, now on the previous page, i.e. page 33. Sorry it has taken me so long to get back to you; other projects intervened.

I'd like to revisit the question of that the earlier post, and perhaps amplify it a bit. If you recall, that question was:

Is a 3-point pivoting sub-frame absolutely necessary? Could we design and construct a box-frame chassis that was torsionally stiff enough, such that it never twists, no matter how irregular the terrain?? Could the vehicle's progressive coil-spring suspension do all the "work" of handling rough terrain, instead?

Ian, you understood the question very well, and thanks again for your response. Thanks in particular for your explanation regarding the Unimogs, which have a chassis that is deliberately designed to twist, a lot. And Victorian, many thanks for your wise, prudent comments regarding the constraints motivating most buyers of expedition motorhomes. Experimentation costs money and time, and most buyers have neither in excess.

You both understood my question, but perhaps many on the forum did not? Even so, I really do want to know whether a "torsion free" chassis -- of the sort that MAN advertises for the SX 45 -- is realistically possible. So to make my question clear to others, below I provide some additional background information, in the first two sections.

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1. MAN TGS

Here is some background for those who might not be familiar with the very large, “8 x 8”, expedition motorhome market segment.

Over the last century MAN has built up a reputation for making rock-solid, very reliable trucks that can function well in off-road conditions like construction sites, or mining operations in the Third World..... And if MAN construction trucks can withstand the rigors of Third-World mining, then -- or so companies like ActionMobil, UniCat, and Armadillo reason -- such trucks can certainly withstand the rigors of off-road (or more like "bad-road") expedition travel.

For examples of some very large, expedition motorhomes based on a MAN chassis, see:

Now as many regulars on the “Overland Portal” perhaps already know, MAN Gmbh has a military division, one that produces off-road military trucks that are widely used by NATO and other armed forces. This is a natural extension of MAN's experience fabricating off-road construction trucks for harsh conditions. As MAN has stated on its military-division website, the expertise it gains via the TGA and TGS line of construction trucks goes into its military vehicles, and so too vica-versa. So a MAN 8 x 8 military truck is, in a sense, a TGA or TGS "in disguise".

The Mercedes Benz Zetros, by way of comparison, is sold mostly to the Bundesweher (the German military), and various armies in Arab countries – see http://www.battle-technology.com/exhibitions.asp?key=653 . As the last link just cited makes clear, over the last few years MAN has delivered literally thousands of new HX and SX vehicles to forces in Europe, the UK, Australia, and New Zealand.

But just in case this SCRIBD link also does not survive, below I've pasted jpg copies of the relevant pages in the PDF.

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3. Is a 3-Point Pivoting Sub-Frame Necessary?

Is 3-point pivoting sub-frame absolutely necessary? That’s the question I asked a few months ago.

According to MAN’s “Mobility Elite” brochure, perhaps not. The following are snapshots from the first few pages of the PDF.

The text in the last snapshot is what got me thinking. Here MAN Military writes:

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"100 % torsional stiffness – the box-type frame.

The extremely torsion resistant box frame with hollow longitudinal members and welded tubular cross members put the SX in a class of its own. With this design the suspension absorbs even extreme terrain irregularities. The desired consequence: the body remains unaffected even during fast off-road driving.

Coil-sprung high mobility – the suspension.

The secret of its high mobility is the progressive coil spring suspension for extremely long spring travel which permits rapid adjustment to the terrain. Additional shock absorbers with integrated dampers on the rear axle are available on request.

A special highlight is the optional hydro-pneumatic suspension with integrated, regulated load-dependent shock absorbers and extremely long spring travel for top driving stability under the toughest conditions. It is equipped with a height adjustment and can be locked in any position."

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Should we take this description at face value? Is this just hype, or is it in fact possible to deliver what MAN Military promises here?

Does it strike those on this forum as feasible to build a frame that is so stiff, that it is 100 % torsion-free, as MAN promises? With a progressive coil-spring suspension that absorbs all“terrain irregularities”, even the most extreme ones? Such that any body placed on top will remain “unaffected, even during fast off-road driving”? If you compare the pictures of the HX and the SX vehicles above, for instance, it does seem that the box-frame in the SX is far more robust. And note that MAN only makes this claim only for the SX-45; it does not make this claim for the HX series.

Also note that MAN repeats this claim in the PDF. In the third-to-last page from the series posted above, MAN writes about the SX-45:"With its unique off-road capabilities and torsional rigidity the SX sets the benchmark in mobility – high performance off-road. It can even get through where only tracked vehicles normally have the capability. The SX is the leader in its class, and either the 3- or 4-axle vehicle is predestined for the transport of high-class, complex and sensitive bodies, ideal as a system and weapon carrier for tactical missions......"

Now those who have been following this thread will realize just how significant a claim this is. MAN is saying that the SX 45 can carry "complex, sensitive bodies" without any need for a 3-point, pivoting sub-frame, precisely because its box-frame proves so torsionally rigid.

*****************************************4. A Real Puzzle

But if this is true, then why hasn't this technology yet "spilled over" from the military side, towards the commercial end of things? Presumably MAN military and MAN commercial do talk to each other? And presumably someone might have seen the utility of a completely torsion-free, "high mobility" box-frame, for makers of very large, expedition-style motorhomes, like ActionMobil, UniCat, and Armadillo?

SX-45 technology has been around for a while: MAN started delivering the first vehicles, it seems, from roughly 2005 onwards. So why are ActionMobil, UniCat, and Armadillo still building their 6 x 6 and 8 x 8 "specials", on frames that twist? If you take a look at some of the videos of 6 x 6 and 8 x 8 vehicles on the ActionMobil and Armadillo websites, you'll see that when they go over rough terrain, as one might expect the cab goes one way, and the body another.... I don't know whether this potential to "twist" was already built into the TGA or TGS construction-vehicle chassis, as delivered from MAN's factory. However it arises, it is certainly a feature of even the largest 8 x 8 expedition motorhomes. It's also quite clear that all of them are still being mounted on 3-point pivoting sub-frames. But given the existence of a potential SX-45-style solution, why hasn't any fabricator yet adopted this as an alternative?

Something here does not quite "fit", and I am not sure what it is. I tried calling up Rheinmettal-MAN a number of times, but just got the run-around, and no-one I talked to seemed to understand my question, let alone provide a straight answer. So this thread seemed like the perfect place to post this puzzle.

Here's hoping that, this time around, many more will understand the query...

But again, Ian and Victorian, many thanks to you both for your responses.

Heretic Car Camper

No such thing as a "100% Torsional Stiffness" chassis. Doesn't exist, never will. All structures will flex. Some can flex significantly before failure, others barely distort before failure, but they all do flex.

What can exist, and possibly does, is a ladder frame chassis that is torsionally stiff enough for the forces that act on it to not twist it.

So is a three point mount necessary? I think that it depends on the base vehicle. With a Mog I think that the answer is an obvious "yes." With other mfg's I'm not as sure. If there is such a thing, I think that they are not necessary with a 'normal' chassis, that simply resting on the top rail with anti-friction methods in place as well as methods that allow the frame rail and the part resting on it to move independently while still securing the payload is all that is needed. To me that is several straps or bolts with compression springs not fully compressed per frame rail.

I think it unwise to try to increase the torsional stiffness of the chassis with the payload. Trying to do so will create a large stress riser at the point of transition from your efforts to the OE's frame.

Designer

Interesting response. So there is an engineering distinction between &#8220;flex&#8221; versus visible &#8220;twist&#8221;? I am not an engineer, but can imagine that there might be such a distinction.

In effect you seem to be saying that perhaps the MAN brochure is not all hype, and it might in principle be possible to create a vehicle that has

You also wrote some interesting things about mounting payloads. But here I wonder whether the system you envision would prove applicable to all the different kinds of payload that the SX45 has been fitted to carry? The last page of the "Mobile Elite" brochure has a few schematics of the possibilities: missile system carrier, bridgelayer, system cabin&#8230;..

The British army, for instance, just received delivery of hundreds of specialized 8 x 8 &#8220;recovery&#8221; vehicles that have mounted cranes. These kinds of things, presumably, would have to be attached to the chassis in a rather &#8220;fixed&#8221; sort of way?

Here again the Panther's "payload", if you will, seems attached to the chassis in a fairly "fixed" sort of way. But granted, on a daily basis, the Panther does not do all that much off-road driving&#8230;..

I am only asking this question because it seems standard wisdom in the Expedition industry that, for larger vehicles, a 3-point pivoting sub-frame is necessary. But then I came across the MAN "Mobile Elite&#8221; brochure, which seems to advertise the SX-45 as not needing anything of the kind.

All best wishes,

Biotect

PS -- Just one practical query. I am still new to the Expedition Portal. How might one upload photographs in such a way that potentially anyone could see them, and not just members who are logged-on? Some of the uploaded images in these threads have a kind of "privacy filter" that prevents outsiders seeing them, while others do not. Mine at present seem to be filtered, but I would like to take the filtering off.

Heretic Car Camper

Twist is a torsional flex, they are essentially the same except in how they manifest themselves.

The mounting method that I suggest has been in use by the U.S. military since at least WWII and possibly earlier. I'm only interested and concerned with how flatbeds and box-vans are mounted since I'm only ever likely to use those types of equipment.

Adventurer

No such thing as a "100% Torsional Stiffness" chassis. Doesn't exist, never will. All structures will flex. Some can flex significantly before failure, others barely distort before failure, but they all do flex.

If there is such a thing, I think that they are not necessary with a 'normal' chassis, that simply resting on the top rail with anti-friction methods in place as well as methods that allow the frame rail and the part resting on it to move independently while still securing the payload is all that is needed. To me that is several straps or bolts with compression springs not fully compressed per frame rail.

My issue with the semi-rigid mount is that as the frame twists it compresses the springs which in turn pull on one or other corner of the payload. It's not torsion free, it just reduces the stiffening effect the payload has on the chassis (see your point below), and reduces the twist the chassis exerts on the payload.
For many applications this is sufficient, as it is a compromise (economical, space, engineering effort etc.) that reduces the transmission from chassis to payload to a tolerable level; for a given payload.
Some payloads cannot even tolerate that, and I personally wouldn't take the risk for an accomodation module. Many do and it works for them, so that is why I reiterate that it is my personal choice not to take that risk.

I think it unwise to try to increase the torsional stiffness of the chassis with the payload. Trying to do so will create a large stress riser at the point of transition from your efforts to the OE's frame.

Designer

This is a design "thesis project" that I will be working on, in earnest next fall, i.e. beginning in September. I've been anticipating a bit, trying to determine the engineering constraints. Please see my first post on page 33 of this thread, for a more thorough explanation.

The basic issue is this: from a purely design point of view, expedition motorhomes seem awfully space-wasteful. In the lingo of the wider RV industry, they are “non-integrated” designs in which a body is simply placed on a truck, whose cab remains separate. So the seating gets duplicated. This contrasts markedly with the most advanced European designs, for instance, Hymer motorhomes. In almost all Hymer RVs, the driver’s seat and passenger seat swivel 180 degrees, and do “double-duty” as dinette seating. Again, please see my post on page 33 for pictures.

Now the interesting thing is that top-of-the-line Hymers are big. Maybe not as big as Class A American motorhomes, but certainly as large as the biggest expedition motorhomes. So if double-duty swivel seating seems worthwhile in a big Hymer, you’d think it would be worthwhile in a UniCat or ActionMobil, too?

As near as I can tell, the main thing that prevents this, is the need to fix cab and body separately, because the latter has to be mounted on a 3-point pivoting frame. Class A American motorhomes and Hymers can be fully integrated designs, it seems, because they don’t need 3-point pivoting frames. And they do not need 3-point pivoting frames, because “regular” RV’s typically do not experience the same stresses as expedition motorhomes.

However, I then came across the SX-45 brochure, and began wondering why this technology had not yet “migrated” over to UniCat or ActionMobil. As stated in the post just above, ActionMobil has already built at least one 8 x 8 expedition motorhome on a MAN KAT military chassis. So perhaps ActionMobil has good enough relations with MAN military, to be able to request a SX-45 chassis? Would it really cost that much more? The engineering has already been done, and perhaps all that MAN needs to do, is combine the SX-45 chassis with the same engine and drive train used by the 8 x 8 TGS – which, by the way, is the chassis used by Unicat in the large build referenced in the previous post; and so too, the chassis used by Armadillo in one of its large builds.

I will be booking appointments with various companies in Germany and Austria, including ActionMobil and UniCat, to do some more direct, “on the shop-floor” research in June. I have two passports, German and Canadian, and speak German reasonably well, so it should be interesting. At present, I am still trying to establish who I should contact at Rheinmettal MAN, the company that has taken over production of the SX-45.

In the meantime, I figured it would be good to post the MAN brochure on the Expedition Portal, and see what the forum’s response would be. Many thanks, Moe, for your enthusiastic reception!

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2. Ntsqd:

You mentioned that the system you described has been used by the American military “since at least WWII and possibly earlier”. I f you have any links to websites that detail visually what you described above, would you be willing to pass them along?

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All best wishes,

Biotect

PS – Again, the practical query: How might one upload photographs in such a way that potentially anyone could see them, and not just members who are logged-on? Some of the uploaded images in these threads have a kind of "privacy filter" that prevents outsiders seeing them, while others do not. Mine at present seem to be filtered, but I would like to take the filtering off.

Designer

Designer

Many thanks for the analysis, and I agree, it may be just a cost issue. But mine is not really a “research” project, in the sense of being just verbal analysis. Rather, it's a design project. I am currently attending design school, so my project will be a concept vehicle, complete with models and illustrations.

Sure, it will be the MFA-exercise of a student. But you might be surprised how much long-term effect some of these exercises can have…. Often transportation designers do their “best” work – in the sense of most innovative – when completing their MFA's. Because once we begin working in the industry, there’s a tendency for thinking to ossify. There is a big market for “independent industrial design studios”, for instance, because companies often find that their own in-house designers simply lack the breadth and distance necessary to create truly innovative products.

Most American motorhome manufacturers, it seems, are forced to offer cabinetry that's best described as "colonial kitsch", with color choices ranging from plaid green to brown plaid, because that's what the American market seems to demand. Just contrast the interior of a typical American Winnebago, for instance, with the interior of a German Hymer. Even some of the Airstreams sold in the United States still offer colonial kitsch cabinetry – see http://www.airstream.com/travel-trailers/classic/photos-decor/ .

But this “market-led” mentality has limits, because often customers don’t know that they would love something different, until they actually see something different. Until Airstream hired Deam, its interiors were just like other RV’s. Deam describes the situation in a NY Times article:

"What I found was, you had this great streamlined aerodynamic modern exterior, and then you opened the door and it was like grandma’s kitchen. There was a disconnect between the exterior and the interior. You approached the trailer and there was the magic promise of the future, and you walk in and it was like a log cabin on wheels. What we decided was, we had to do some kind of archaeology, stripping it down and getting rid of all the gewgaws and clunky interior, and taking it back to something really essential. I simplified it and emphasized the horizontal lines and put in a lot of fluid, curved laminates." - http://www.nytimes.com/2012/02/23/garden/christopher-deam-brings-airstreams-interior-up-to-date-qa.html?_r=0 .

As you might appreciate there’s also an economic payoff, because Airstream has now achieved even more “product differentiation” within the saturated American RV market. Airstreams were already a rather unique product because of their exteriors. Now (most of them) are also unique because of their interiors, too.

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2. The 1990’s “haute-IKEA” aesthetic of German Expedition RV’s

The world of really big, off-road-capable RV’s is, for the most part, German – pace Earthroamer, GXV, Tiger, etc. As such, the interiors of UniCats and ActionMobils have not resembled grandma’s colonial-kitsch-kitchen for quite some time. But even still, in comparison to the wider German RV industry, UniCat and ActionMobil interiors do seem a bit behind the times.

So cost is not the only constraint, it would seem. Either you have the designers and craftsmen who can pull off a Hymer interior, or you don’t. I suspect that ActionMobil and UniCat could have more interesting, up-to-date, curvilinear interiors, if customers began demanding as much. But until customers do, their interiors will continue to look like what they are, namely, interiors designed mostly by engineers.

*****************************************

3. Courageous Color, ARC, and Art Deco

My other quibble with the RV industry in general, is that it plays things rather “safe” when handling color. If one compares the interiors of all of the above brands – European and American, regular RV and expedition RV – to the new Airstream interiors, one huge, glaring difference becomes obvious: the color choices of most motorhome manufacturers tend to be muted, a restricted palette of whites, greys, browns, and blacks. Whereas Airstream, since it hired Christopher C. Deam, has developed the courage to handle strong, vibrant interior color, and a dramatic mixture of materials.

What really impresses me about ARC is their bold color choices (complementaries like green and red, yellow and purple); their mastery of curvilinear, aluminum-detailed cabinetry; the asymmetrical layouts; and the retro details throughout – port-hole windows, jet-engine spot-lamps, and circular-grilled air vents, for instance.

For me, Art Deco is the modernist design-aesthetic that should have dominated the 20th century. But for whatever reason, lots of people found the puritanical-minimalist, black-white-grey, Bauhaus rectilinearism of Mies-and-co ever so convincing, perhaps because it was so cheap on the details and colored paint? Remember, I am German, so perhaps I am “allowed” to take a swing at Mies and the Bauhaus…. The 1990’s/early 2000’s rectilinearism of IKEA is then the direct descendant of the Bauhaus aesthetic, as are the UniCat and ActionMobil interiors.

However things have changed dramatically in architecture and design since the late 1990’s, when “organic-tech” first became fashionable. And as already demonstrated above, German RV manufacturers like Wesfalia, Hymer, Euramobil, etc. have been offering more curvilinear interiors for quite a while. One could even say that an Art Deco/Streamline aesthetic never really disappeared, persisting in certain design niches, transportation design in particular. If only because rectilinear, squared-edged vehicles are not that functional, i.e they're not aerodynamic.

So let’s just say that, even though UniCat and ActionMobil are not building grandma’s colonial kitchen, they are now at least 2 decades behind the times, vis-à-vis wider design trends. Their engineering is no doubt top-notch, and Victorian quite rightly defended UniCat's engineering and careful craftsmanship in an earlier post. But just take a look at the links above, check out Wesfalia, Hymer, Euramobil, et al, and everything that I have written here will seem kind of obvious.

*****************************************

4. Back to Engineering

Now don’t worry, I realize that the bulk of this post was very off-topic, because this is an engineering thread about 3-point pivoting sub-frames. But just thought I should state all the above, so there’s no mystery as to where I am “coming from”. I am a designer, who wants to “prod” the expedition-RV industry a bit, with a concept vehicle that breaks a few unwritten rules.

For instance, my suspicion is that companies like ActionMobil and UniCat are still mounting camper bodies on 3-point pivoting sub-frames mainly due to inertia, and not cost. And they are still doing 1990’s-style “haute-IKEA”, because customers have not demanded more. It sort of takes an outsider like me to come along and say, “Hey, why couldn’t you do x, y, or z instead?”

By far the best way to do that, in the world of design, is by presenting alternative imagery.

Designer

Many thanks for the analysis, and I agree, it may be just a cost issue. But mine is not really a research project, in the sense of being just verbal and quantitative analysis. Rather, it's a design project. I am currently attending design school, so my project will be a concept vehicle, complete with models and illustrations.

Sure, in the end it's just the MFA-exercise of a student. But you might be surprised how much long-term effect some of these design exercises can have…. Often transportation designers do their best work – in the sense of most innovative – when completing their MFA's. Because once we begin working in the industry, there’s a tendency for creativity to ossify. There is a big market for independent industrial design studios, for instance, because companies often find that their own in-house designers simply lack the breadth and distance necessary to create truly innovative products.

Now to be sure, most American motorhome manufacturers seem forced to offer interiors that are best described as "colonial kitsch", with color choices ranging from plaid green to brown plaid, because that's what the American market demands. Just contrast the interior of the typical mid-market American motorhome with the interior of a German Hymer. Even a few Airstreams sold in the United States still offer colonial kitsch cabinetry – see http://www.airstream.com/travel-trailers/classic/photos-decor/ :

But this market-led mentality has limits, because often customers do not even know that they might like something different, until they actually see something different. And until Airstream hired Christopher C. Deam, Airstream interiors were not that different. Deam describes the situation in a NY Times article:

"What I found was, you had this great streamlined aerodynamic modern exterior, and then you opened the door and it was like grandma’s kitchen. There was a disconnect between the exterior and the interior. You approached the trailer and there was the magic promise of the future, and you walk in and it was like a log cabin on wheels. What we decided was, we had to do some kind of archaeology, stripping it down and getting rid of all the gewgaws and clunky interior, and taking it back to something really essential. I simplified it and emphasized the horizontal lines and put in a lot of fluid, curved laminates." - http://www.nytimes.com/2012/02/23/garden/christopher-deam-brings-airstreams-interior-up-to-date-qa.html?_r=0 .

As you might appreciate, there is a potentially big economic payoff here, because Airstream has now achieved even more product differentiation vis-a-vis the saturated American market for caravans. Airstreams were already a rather unique product to begin with, because of their exteriors. Now (most of them) are also unique because of their interiors, too.

*****************************************2. The 1990’s “haute-IKEA” aesthetic of German Expedition RV’s

The world of really big, off-road-capable RV’s is, for the most part, German – pace Earthroamer, GXV, Tiger, etc. As such, the interiors of UniCats and ActionMobils have not resembled grandma’s colonial-kitsch-kitchen for quite some time. But even still, in comparison to the wider German RV industry, UniCat and ActionMobil interiors do seem a bit dated.

So cost is not the only constraint. Either you have the designers and craftsmen who can pull off a Hymer interior, or you don’t. I suspect that ActionMobil and UniCat could have more contemporary, curvilinear interiors too, if customers began demanding as much. But until customers do, their interiors will probably continue to seem driven mostly by engineering considerations.

*****************************************

3. Courageous Color, ARC, and Art Deco

My other quibble with the RV industry in general, is that it plays things rather “safe” when handling color. If one compares the interiors of almost all of the above brands – European and American, regular RV and expedition RV – to the new Airstream interiors, one huge, glaring difference becomes obvious: the color choices of most motorhome manufacturers tend to be muted, a restricted palette of whites, greys, browns, and blacks. Whereas Airstream, since it hired Deam, has developed the courage to handle strong, vibrant interior color, and a dramatic mixture of materials.

For me, Art Deco is the modernist design-aesthetic that should have dominated the 20th century. But for whatever reason, lots of people found the puritanical-minimalist, black-white-grey, Bauhaus rectilinearism of Mies-and-co ever so convincing, perhaps because it was so cheap on the details and color paint? Remember, I am German, so perhaps I am “allowed” to take a swing at Mies and the Bauhaus…. The 1990’s/early 2000’s rectilinearism of IKEA is then the direct descendant of this Bauhaus aesthetic, as are most UniCat and ActionMobil interiors.

And as demonstrated above, German RV manufacturers like Wesfalia, Hymer, Euramobil, etc. have been offering more curvilinear interiors for quite some time. One could even say that the Art Deco/Streamline aesthetic never really disappeared, persisting in certain design niches, transportation design in particular. If only because rectilinear, squared-edged vehicles are not very functional, i.e they're not aerodynamic.

So let’s just say that, even though UniCat and ActionMobil are not building grandma’s colonial kitchen, they are now roughly 2 decades behind the times, vis-à-vis wider design trends. Their engineering is no doubt top-notch, and Victorian quite rightly defended UniCat's careful engineering and craftsmanship in an earlier post. But just take a look at the links above, check out Wesfalia, Hymer, Euramobil, et al, compare for yourself, and everything that I have written here may seem somewhat self-evident.

*****************************************

4. Back to Engineering

Now don’t worry, I realize that the bulk of this post was very off-topic, because this is an engineering thread, about 3-point pivoting sub-frames. But just thought I should state all the above, so there’s no mystery as to where I am coming from. I am a designer, who wants to poke the expedition-RV industry a bit, with a concept vehicle that breaks a few unwritten rules.

It's easy enough to create wild designs that have no basis in engineering reality, and the world of transportation design is littered with concept vehicles that never got built, because they can't be built -- at least not cost-effectively. It's much harder to think through the incremental changes that are much less dramatic, but that, over time, can cumulatively revolutionize a given vehicle-type.

For instance, my suspicion is that companies like ActionMobil and UniCat are still mounting camper bodies on 3-point pivoting sub-frames mainly due to inertia, and not just cost. I may be wrong about this, and this needs investigation. But it's at least a possibility. A fully integrated off-road mobile home would be highly desirable, and perhaps not that much additional engineering is necessary to pull it off. This might be an "incremental" change, from an engineering point of view, but one that would change the game dramatically from a design point of view, with significant consequences for overall interior plans, layouts, the efficient use of limited space, etc. Similarly, I suspect that ActioMobil and UniCat are still doing 1990’s-style “haute-IKEA”, perhaps because customers have not demanded more. It sort of takes an outsider like me to come along and say, “Hey, why couldn’t you do x, y, or z instead?”

By far the best way to do that, in the world of design, is by presenting alternative imagery.

Adventurer

As KAT 1 owner I can say that the frame isn't totally stiff. This is also the reason why the load platform of the military base is fixed by springs and not bolted or fixed by screws.

But it is stiff enough so that springs are good enough even in the worst off-road situations. But even then I put my cabin on a 4-point pivoting fixture with the distribution of the force over are large segment of the frame, in order to keep the expensive cabin away from any structural stress. This has worked perfectly during the last 7 years. Some people have fixed their pivoting fixture only at a few points and got damage at the L frame sitting on top if the closed master box frame. This shows that there are considerable forces at work.

The normal ladder frames used by MAN LE/TGS/TGA and others are very flexible in contrast. This has to be in combination with leaf springs to get acceptable offroad capabilities. This soft frames work fine in the typical use scenario of a few 1000 mls in construction environment during lifetime. But if you drive 10.000s of mls on bad roads with constant torsion caused by uneven road and swinging load on top, the risk that something breaks at the ladder frame is high. There have been several reports about frame damage of LE models after driving many miles on the bad roads in South America. Of course, it also depends on the load and speed you drive. Very often such camper trucks are constantly near their upper weight limit.

The Tatra central frame tube of T813, T815, ... is even stiffer than the frames of MAN KAT and SX military trucks. But it is also much heavier.

The reason why the MAN stiff frame trucks are not used more widely is that they are military technology with low production count. No one would spend more than US$ 300.000 new for the base truck when the regular ones cost only 1/3. The military trucks are only affortable used when about 20-30 years old. Currently the KAT I 4x4 can be bought from Vebeg for much less than US$ 10.000, because they are more than 30 years old.

In the user manual of my KAT I 6x6 you can read the following description:
The truck 7t mil gl is a transport vehicle that is able to follow tanks (chain or wheel).

This is shown at every point of the construction. I have driven bad gravel roads in Russia with 50+ mph where other road vehicles drove only 10-20 mph. The spring suspension gives a very comfortable ride. In Schweden I once forgot to put the cap of the engine oil refill on top of the flat oil coolers of the engine. After driving about 5 miles on gravel road to a recreation area, I remembered that I have forgot something and found it still at the same place.

Regarding design, you can ask 10 people and get 20 different opinions. In my opinion if someone has serious interest in going on a large trip away from civilisation a simple functional design wins hand down. Using to much curvature will loose valuable space inside the box. It makes just no sense to have a lot curvature at the outside when the size limits are rectangular. The so called modern designed interior and exteriour is more destined to show how rich and wealthy you are. Driving with such a luxury expedition vehicle through 3rd world countries is not the best way to get in touch with friendly local people, it is more a way to get in contact with bad people. Understatement is a better way here. And most people buying such vehicles from the established market leaders understand this. The people that look for top design in such vehicle will finally fail to go on any long voyage and the vehicles are for sale very soon.

Designer

So even when used in heavy mining work, even as loader-tippers, MAN TGS frames are still flexible?

From your description, it does seem that a very rigid chassis with progressive coil suspension is the best way to go, as opposed to a more flexible chassis with leaf suspension. This is basically the difference between the SX and HX series, it would seem. And of course it's the difference between the SX and commercial TGS trucks.

So, given a choice between a 10 or 20-year old used KAT 1 chassis, versus a new TGS chassis, the used military chassis is the better choice, right? Because it will be stiffer, and will have progressive coil suspension?

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You said that your KAT 1 frame is not totally stiff, and MAN's product literature seems to suggest that the HX series is also not totally stiff either. MAN military only promises 100 % torsional stiffness for the SX-45.

But do you think the SX-45 would be stiff enough to completely eliminate the need for a 4-point pivoting sub-frame? Or do you think a pivoting sub-frame is still needed, even for the SX-45?

Furthermore, does the SX-45 also need a load platform fixed to the chassis base with springs, and not bolted or fixed by screws? As per your KAT 1? Clearly, such a spring-loaded platform is needed for the KAT 1 and the HX. But do you think it is also needed for the SX-45?

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It seems hard to find more detailed information about the SX-45 on the web. I've tried to find the proper contact person at Rheinmetall MAN, but so far, no luck. So if you know of any really good, informative websites (auch in Deutsch!), websites that detail SX-45 payload mounting, would you be willing to pass along the links?

Finally, do you think a camper body could be directly mounted on a Tatra T813 or T815 frame? Or, even then, would you still strongly recommend a spring-loaded base platform, and/or a 4-point pivoting sub-frame on top of that?

Adventurer

The KAT I is the first truck of the military truck series of that lead to the SX series.
- KAT I 4x4, 6x6, 8x8
- KAT II 8x8 build for the US Army
- KAT I A1 with Deutz 513 engine
- KAT I A1.1 with MAN engine and hydraulic suspension
- KAT I Multi
- SX-Series
There were also some Prototypes X-number in between, which were the base of many airport fire trucks build by Rosenbauer and Ziegler. To reduce development and production cost over time MAN put more and more parts from the civil production lines.

The HX is based mainly on civil components, except for the modular cab. Before the HX there was also a cheaper "light" KAT called KAT III LX with leaf springs build for some other countries.

For fast offroad driving in my opinion the stiff frame coil suspension combination will always be better then the bending frame leaf spring combination. But it also has disadvantages in some situations where the range of the suspension is not large enough. The load platform of a KAT is much higher than with other vehicles as there has to be enough room for the subended wheels. . This high center of gravity does allow only a limited side angle until it will tip over. This has been "fixed" in some way by introducing the hydraulic controlled suspension HEPLEX starting with A1.1 model.

As far as I know the SX has the same frame as the KAT and therefore isn't 100 % stiff, just as the KAT. But you don't need a pivoting suspension for a load, just some springs to fix any load.

I would even allow some flex with the Tatra, if the vehicle is going really offroad. When a frame is about 10 m long there will be always some flex that has to be compensated, apart from pure extension or contraction caused by temperature changes.

I had once access to MANTED the MAN modell and construction database, but I cannot remember if it contained information about the SX series. But you may post your questions in the german KAT forum. There are a lot of knowlegable people around there that may know all the details you need.